The first time Castiel is ordered to kill Dean, he puts up a fight. He slashes his blade at the angels around him, the ones forcing him to do this to his best friend. He shouts and fights and he beats them off with his wings, desperate not to do this. His knuckles are sore and bloody, he kicks and bites and he pleads and begs not to do this. Not to have to kill Dean.
They swarm him, more than enough to subdue him. He's beaten until he can barely breathe. His skin is bruised and his bones are cracked and he is so close to giving up and just letting them kill him. Their iron fists against his soft human vessel. The pain is overwhelming, yet it's dull compared to the horror he's feeling.
The fists stop and the angels disappear. They're just... Gone. As if they were never there. Naomi enters the room and takes his hand, pushes the blade into Dean's chest. Cas can't see through his tears, can't hear her congratulations through the pitiful wailing from him.
The second time Cas has to kill Dean Winchester, he's guided through it by Naomi herself. This is how it goes from there, over and over his hand is forced down on Dean. He's still crying, tears are falling from his eyes even after the twentieth time.
Each time he thinks about it, each time he falls to his knees sobbing, there's another Dean to kill. Castiel is given no respite. He is forced, over and over, to kill his best friend. He is forced to watch the light in Dean's eyes go out. The clones of Dean, these bodies that litter the floor, they're the closest he'll get to Dean again. He's sure of it.
By the twenty-fifth time Naomi has stopped guiding him. She stands and watches him. Cas' movements are slow, apologetic almost. He gives Dean a quick death and falls to his knees. He is crying, bringing his hands to his face and letting silent tears fall. The sound is heard throughout the building; it's the sound of a heart breaking.
Somewhere about the fortieth time Castiel slides the blade into Dean, but his hand is stilled when he remembers what Dean had done for him. He can't complete the murder. He cradles Dean in his arms as the Winchester dies, his breathing slowing down and his face becoming less and less Dean-like. Castiel begins crying again. His shoulders shake as the tears cascade down his cheeks.
"Oh God forgive me. Forgive me for what I've done. I never meant to hurt him. I didn't want to kill him." Over and over the angel begs his father to forgive him.
About the sixtieth time, Castiel stops feeling things. Something has changed within him. Something is not the same. This is not the angel that was brought in days ago. This is something new, and something old. This is a solider, an angel of The Lord.
By the hundredth time, Castiel is killing without thought. Quick, mechanical movements. He strikes Dean down and he does nothing to show that he is feeling anything. His eyes are dead, his hand still grips the angel blade, his breathing is regular.
Around about the four hundred and thirtieth time, Castiel attempts suicide. He presses the angel blade to his chest and looks up at the droid of Dean, a prayer on his lips. He pushes the blade into his chest.
He wakes up a few days later, still in the warehouse. Naomi is stood in front of him. She is angry, and disappointed. "Castiel, I thought we fixed you."
"I- I'm not broken." Castiel mutters back, regaining some feeling in his body.
"Oh, but you are." Naomi croons, "Broken in ways even I can't fix."
Castiel blinks at her. He doesn't understand. How is he broken?
Naomi hands him a blade and disappears.
A few seconds later Dean walks though the door, his gun is loaded and he is on edge. His hair is messy and he's breathing heavily. "Cas?" He calls out into the darkness. He is blinded by the darkness of the building, Cas learnt that several hundred Deans ago. Castiel stands up and walks straight towards Dean, he places on hand on his forehead and rams the blade into his stomach.
Castiel resumes killing Dean. If he cannot kill himself, if he cannot even do that right, what right does he have to judge the almighty's will? He is nothing. He is an angel of The Lord, not designed to think. He is only designed to follow his father's orders. He is not Lucifer. He is Castiel, he is destined to do this.
It is about the nine hundredth Dean that Naomi returns. She looks into Cas' eyes and a shudder runs through her spine. She cannot believe that this - this soulless being, this shell of an angel, this beast - is what Castiel becomes without Dean Winchester.
Castiel cannot tell the difference between Naomi and Dean any more, they're one and the same in his mind. Over and over he has visualised killing Naomi, only to see Dean dying in his arms. He cannot stop his arm from rising and his blade from arching down over Naomi.
She parries the blow and shouts at Castiel. The building shakes, Castiel falls to the floor. Even Naomi herself is shaken. Castiel stands up and looks at his superior, willing to accept his punishment.
Naomi shakes her head and leaves Castiel alone again. It has been several weeks since he last spoke to anyone, Naomi is the only person he sees other than the Deans he kills. She is his only link to the outside world.
Castiel kills over a thousand Deans before Naomi is happy.
Castiel cannot remember what it was like to be happy. He only remembers the mind numbing pain of having Dean beg him for mercy, of Dean's eyes closing as he was reflected in them.
Castiel cannot remember what it was like to laugh, cannot remember what it was like to do anything but follow his orders. He cannot remember anything but the mechanical killing of Dean Winchester.
Naomi sends him out into battle. Castiel is not Dean's Cas any more. He is Castiel, angel of The Lord and destined killer of Dean Winchester